1. Field of the Invention
The present invention relates to image processing devices, image display devices, and image processing methods, that interpolate tones of digital image data, in particular, to curb generation of a pseudo contour in the digital image.
2. Description of the Related Art
In recent years, display devices such as televisions and monitors have been made brighter. Making the devices brighter increases the amount of luminance change per tone, causing gradual tone changes to become noticeable. Displaying devises with high resolution in tone, such as a 10-bit liquid-crystal-display panel, have been developed; however, because images are distributed in 8 bits in digital broadcasting and the like, an effective number of tones remains in that expressed by 8 bits, despite the 10-bit tone resolution of the display device. Therefore, at least three image levels are lacked, then in an image—a sunset, a sea and the like whose tone changes gradually, the image level changes stepwise across its neighboring areas, which look like pseudo contours.
When performing a smoothing process (such as a simple averaging) of image data that includes pseudo contours caused by lack of image levels, there arises a problem in that the image becomes blurred. For example, in a method of interpolating tone as set forth in Japanese Patent Application Publication No. H10-84481 (Patent Document 1) defined as a “tone jump” is a phenomenon in which the image level changes stepwise across neighboring areas so that at lease one image level is lacking. In technologies disclosed in Patent Document 1, in order to eliminate the tone jump without performing a smoothing process, a target area where tone is to be interpolated is extracted by identifying a first and a second image levels between which a tone jump exists, and the target area is portioned into N+1 segments, so that N intermediate image levels are assigned sequentially to interpolate tones of the target area.
However, in the techniques disclosed in the Patent Document 1, tones are interpolated based on data in narrow areas, causing a problem in that pseudo contours produced from gradual changes cannot be eliminated.